Organopolysiloxane oils are used as energy transfer media in hydraulic systems, heat transfer systems and viscous or fan couplings. They are subjected to great stresses such as high shear forces, high temperatures and abraded metal. The characteristics of the devices must not change in response to any of these stresses; thus despite the viscosity changing owing to shear and temperature, the transmitted torques should remain as constant as possible. On no account should the organopolysiloxane oil gel, because gelling of the organopolysiloxane oil would eventually lead to failure of the system. For this reason commercial organopolysiloxane oils, which are desirable because of the low temperature dependence of their viscosity, their good thermal stability and the ease of preparing them with any viscosity, are not suitable for sustained use since they usually gel. For these applications, specific additives are needed as stabilizers.
As such additives, EP-A-191 456 describes zirconium-containing organosiloxanes together with cerium-containing organosiloxanes; U.S. Pat. No. 5,013,858 describes mixtures of anthraquinone compounds with polymethylhydrogensilanes terminated by trimethylsilyl end groups; EP-A-397 507 describes phosphorus-containing compounds to which sulfur compounds such as di- and trisulfides or zinc dithiophosphates can also be added, or metal deactivators and/or corrosion inhibitors, and U.S. Pat. No. 4,959,166 describes hydrocarbon sulfide, the reaction product of oil or fat with sulfur, a zinc dithiophosphate, a metal salt of dialkyldithiocarbamic acid, elemental sulfur and a cyclic hydrocarbon sulfide.
The additives described improve the stability of the organopolysiloxanes, specifically their stability to gelation, only insufficiently.